I used to have two on my Kookaburra Rogue, and would recommend that - as a general rule of thumb - two is the minimum amount to use, especially if they are thin grips. Not only does it pick up better, but the ping feels so much nicer, and you don't get much - if any - of those horrible vibrations when you toe end the ball.
However, on my current weapon of choice (Salix Praestantia Performance), I only find myself needing the one as it comes with the lovely thick Spiral grip. So, for me, it really depends on the thickness of the grips and they vary a lot from grip to grip...